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LES OF TRANSIENTS IN THEFRANCIS-99 WATER TURBINE MODEL
JONATHAN FAHLBECK1, LUDVIG UPPSTRÖM2,ERIC LILLBERG3, HÅKAN NILSSON4
1MSc student, Chalmers University of Technology, Mechanics and Martime Sciences, [email protected]
2MSc student, Chalmers University of Technology, Mechanics and Martime Sciences, [email protected]
3Vattenfall AB Research and Development, [email protected]
4Chalmers University of Technology, Mechanics and Maritime Sciences, [email protected]
13th OpenFOAM Workshop, Shanghai, 2018-06-25--28
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Introduction and outline
• Water turbines are more and more used to stabilize the electric grid, both in terms of power availability and frequency (50Hz).
• Off-design operation, start-stop, varying conditions, speed-no-load
o Hazardous flow-induced instabilities
o Optimal operation needs to be determined
• The Francis-99 turbine model is in the present work used to:
o Validate at best efficiency
o Study a full shut-down sequence (and four start-up initiations, if time allows)
▪ Involves changes in guide vane angles while the runner is rotating
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Francis-99 geometry
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Top View Side View
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Experimental probes and velocity lines
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Numerical setup
• Turbulence modelling
o LES and dynamic one-equation eddy viscosity model
o Cube root volume (3𝑉)
• PIMPLE for pressure-velocity coupling
• Boundary conditions:
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Numerical setup
• Mesh:
o Four parts, SC-SV, guide vanes, runner and draft tube
o CyclicAMI rotor-stator coupling
o 20 million cells in total
• Runner mesh motion (BEP and transients)
o Solid-body rotation, 333RPM
• Guide vane mesh motion (transients)
o Discussed later…
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BEP: Pressure and velocity fields
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BEP: Static pressure• Mean static pressure validation
• From 1st Workshop (except VL2/DT5, from 2nd)
• VL2 Perfect match: 0.4 % error
• Fluctuating pressure in DT
• Magnitude match
• Almost same standard deviation
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BEP: Velocity profiles, Line 1
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Shut-down simulation
• BEP (9.84°) to almost closed (0.80°)
o Guide vane closing speed 1.3°/s
o Start at t = 1 s
o Finished at t = 8 s
• Guide vane mesh motion (transients)
o Displacement laplacian mesh morphing
o Initial mesh at BEP (9.84°)
o Map to new meshes at 4.67°, 2.00° and 0.80°
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Guide vane meshes
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9.84° (BEP)
cfMesh
4.67°
cfMesh0.80°
snappyHexMesh
2.00°
cfMesh
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Shut-down: Velocity magnitude
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9.84°, BEP 6.59° 5.55°
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Shutdown: Static pressure
15VL2 DT5
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Shutdown:Velocity at a point, Line 2
16Vertical Horizontal
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Shutdown:Velocity evolution, Line 2
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Numerical Experimental
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Shut-down: Q-criterion• Stay vane horseshoe vortex
• Formation of vortex rope
• Vortex rope break-down
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9.84°, BEP 6.54° 5.37° 4.33°
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Conclusions
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The results are very promising, but there is room for improvement (at least):
• Mesh generation (easier, better, specialized for turbomachinery,
in particular when almost closed GV)
• Mesh deformation functionality, accuracy and efficiency
o Deformation in only part of the domain, while other parts are
given (e.g.) a solid body rotation (subSetMeshMotion fails in parallel)
o Mesh slip on general geometric surfaces
• Inclusion of cavitation
• Computational speed (always)
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Thank You For Listening!
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Questions?
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Start-up simulations
• Investigate difference in guide vane opening speed
• Four short start-up cases
o 0.975°/s
o 1.30°/s (as for shot-down)
o 1.95°/s
o 2.60°/s
• Snapshots at α = 2.36°
• Plotted against guide vane angle
• Compared with experimental data
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Start-up: Q-criterion at α = 2.36°
• Large similarities
• More turbulent for the slower case
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Start-up: Velocity magnitude at α = 2.36°
• Velocity focused to the walls at faster speed
• Seems more turbulent for a slower case
o Had more time to build up vortex
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Start-up: Velocity contour• Larger unsteady behaviour for the slower
• See at the walls
• A smoother change for a faster case
• Note: Now plotted against guide vane angle!
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Start-up: Pressure and velocity
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Static pressure Vertical velocity at point, Line 2
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Start-up: Force and torque
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Axial force Axial torque
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Start-up: Frequency spectrum
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VL2 P42
